Relationship of plasma and energetic particles in geomagnetic tail to particle precipitation into auroral zon

Anderson, K. A.

English

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sJuly, 1968
I
RELATION OF ENERGETIC PARTICLES IN THE
PLASMA SHEET TO THE AURORAL ZONE-p1,
K. A. Anderson
Physics Department and Space Sciences Laboratory
University of California
Berkeley, California
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*This work supported in part by National Aeronautics and Space
Administration Contract NAS5 -90?? and Grant NGL 05-003-017
2.
This article will review the observational evidence for a rela-
tionship between plasma and energetic particle phenomena in the gv.o-
magnetic tail and precipitation of particles into the auroral zone. First,
the properties of the plasma and energetic particles in the geomagnetic
tail will be summarized. A fundamental feature of this region is
that it contains a thick slab of hot plasma extending symmetrically
northward and southward from the region of field reversal (neutral
sheet). The total thickness of the plasma sheet is several earth radii
(— 30 0 000 km). The plasma sheet thickness is probably variable and
the whole sheet at times may move rapidly with respect to a coordinate
system such as the solar-m,,^gnetospheric. The plasma sheet extends
across the tail to the mE.gnetopause and back into the tail to at least
distances of 60 earth radii. The plasma particles are highly variable
in energy. At a geocentric distance of 18 R e the peak of the electron
energy spectrum ranges from 0.1 to 10 KeV and in the case of the pro-
tons the range is 1 to 20 KeV. The proton energy density is about 8
times that of the electrons.	 The particle density in the plasma sheet is
about 1 cm -3 . This information on the plasma properties comes from
the VELA satellite and is summarized by BAME (1968). VASYLIUNAS
(1968) has extended the plasma sheet measurements in closer to the
earth. He finds that the plasma sheet extends past the dusk meridian
into the sunlit hemisphere. At a Gordon Conference ( 1968) he showed
evidence that the plasma sheet extends to low altitudes on the night side
of the earth close to or into the auroral zone. The spatial extent of
the plasma sheet and its relationship to the auroral zone is shown in
Figure 1. BAME et al (1967) have shown that the plasma may suddenly
become strongly heated in the sense that the average particle energy is
F `,
Lim
3.
greatly increased. It is at these times that the energetic (-. 20 KvV)
fluxes of electrons appear, then slowly decay away. The iunpulsi ^ c"
acceleration events resulting in electron island fluxes v;as first
reported by ANDERSON, HARRIS and PAOLI ( 1965) and their proper-
ties were more fully discussed by ANDERSON (1965), MONTGOMERY
et al ( 1965) and KONRADI ( 1966). Many examples of this phenomenon
may be found in those articles. Figures 2 and 3 show additi , ial
examples. ANDERSON and NESS (1966) showed that the island fluxes
appear in a slab-like region straddling the neutral sheet in which the
magnetic field was markedly weakened; an effect they attributed to the
diamagnetism of a plasma in this region. There is now little doubt
that this region is just the plasma sheet discussed by BAME et al
(1967) and by VASYLIUNAS (19613). Some of the known properties of
the island fluxes are summarized in Figure 4.
A resemblance between electron island fluxes occurring in the
tail and aurora] zone phenomena has been recognized for some time.
This resemblance can be seen by comparing Figure 5 with Figures 2
and 3. In Figure 5, balloon measurements of X-rays due to the
bremsstrahlung of electrons with energies > 25 KeV are seen to exhibit
fast-slow behavior with characteristic times about the same as island
fluxes in the tail. Also at the time of the electron precipitation, an
intense negative bay occurs showing that an auroral substorm is in
progress.
HUDSON (1966) has studied the energy spectrum of the electrons
coming into the auroral zone during auroral breakup. In the few cases
measured, he found a progressive softening of the spectrum from an
e-folding energy of 10 KeV to about 5 KeV. This is just the behavior
of energetic electrons in the tail during island events as shown in
Y	 ^.
4.
Figure 4. This suggests that one of the phenomena that occur during
the complex substorm process is precipitation of particles out of t1w
plasma sheet region into the atmosphere.
Several attempts have been made to more closely relate pheno-
mena occurring in the tail with effects in the auroral zone. PARTHA-
SARATHY and REID (1967) and DRIATSKIY (1967) studied the ionospheric
effects produced by precipitating particles and associated them with the
appearance of energetic electrons deep in the magnetotail. The combined
results for an event which occurred on 28 May 1964 are shown in
Figure 6. The Russian arctic stations were at local tines hecween
0200 and 0300 at the appearance of riometer absorption and the South
Pole station was also within a few hours of magnetic midnight. It is
interesting that the electron flux actually observed in the tail on this
occasion was comparatively weak. Many islands have been observed
with much higher intensity. However, it is likely that an intense plasma
sheet acceleration event (island) did take place at this time. The space-
craft was somewhat to the south of the neutral sheet region at this
time and there had been no detectable electron fluxes > 40 KeV for
the previous day. Evidently a portion of the plasma sheet suddenly
filled with energetic particles and expanded, enveloping the spacecraft.
It seems likely therefore that had the spacecraft been near the center
of the plasma sheet much greater electron intensities would have been
observed. The island event occurs near the beginning of the riometer
effect possibly meaning that the plasma sheet has relaxed back to its
former configuration. DRIATSKIY (1967) also reported a close corres-
pondence between riometer absorption and appearance of electron fluxes
deep in the magnetotail on 14 May 1964. On the other hand he reported
occasions when well-developed island fluxes were not associated with rio-
meter effects.
- - ►
 -	 - - -
	 M
5.
ROTHWELL (1967) has compared electron measurements in the
magnetotail with bay-type disturbances in northern hemisphere mag-
netometers and finds the general correlation that wK?n island activity
is high in the magnetotail the chances are high that a negative bay
will occur at sites near local magnetic midnight. Ar.., example from
more recent satellite measurements confirming this result is shown
in Figure 7.
Questions that arise concerning the origin of plasma sheet
acceleration events are: (1) Do particles become accelerated in the
tail, then propagate into tha auroral zone? (2) Alternatively, are
particles accelerated much closer to the earth (L — 3 to 8) then move
out into the tail region? (3) As a third possibility, does an instability
excite a propagating disturbance which generatPf charged particles as
it moves along? (4) If the latter is the case, in which direction does
the disturbance move?
Various attempts have been made to answer these questions. If
hydromagnetic disturbances play a role they should be recognizable from
the rather long delay times involved. The Alfven speed is 	 00 km/sec
in the tail so that about 15 minutes are needed for a disturbance to
propagate over a distance of 30 earth radii. HONES et al (1967) have
looked for this time delay using Vela satellite data and ground magne-
tometers. They first of all find the general correlation discussed
above that frequent plasma sheet acceleration events implies frequent
magnetic bays for stations near local midnight. The appearance of the
energetic electrons at the spacecraft situation at 18 Re geocentric
distance was found to occur coincident with or following the deepest
portion of the bay decrease. This result applies to situations when it
was believed that the ground magnetometer and the spacecraft were
-
V.
connected within a rather small tube of geomagnetic field lines. This
result appears to favor the view that an h-m disturbance travels out-
ward into the magnetotail from an origin inside 18 R e geocentric distance.
No consistent value for a delay time, i.nd hence a velocity, could be
determined, however.
A different way of studying the questions raised above is to
determine the peak intensity of plasma sheet acceleration events as a
function of distance into the tail. The result from one such comparison
is shown in Figure 8. The IMP-3 data were taken in the geocentric:
distance range 20 to 40 Re while the EXPLORER-35 data all come
from close to 60 Re . The IMP-3 results were obtained uniformly
across the tail while the EXPLORER-35 results are biased somewhat
toward the dawn side of the tail. Kp conditions are about the same for
both sets of data.	 A large radial effect is Seen in Figure 8. Appa-
rently the acceleration process is weakening as a function of distance
into the tail. The dawn-dusk asymmetry reported by BAME et al
(1967) if it exists as far back as 60 R e , would serve to make the
radial dependence effect even greater than shown in the Figure. The
number of acceleration events per unit time above a very low flux
threshold does not change greatly as a function of distance into the
tail. ANDERSON (1965) previously reported a radial dependence of the
number of islands with distance into the tail above a threshold of about
10 5 cm- 2 sec -1 > 40 KeV electrons. This can now be understood as a
constant rate per unit time of island events down to flux values as .low
as 10 3 cm-2 sec -1.
The h-m waves producing the large modulation of the energetic
electron fluxes are a prominent feature of the skirt and plasma sheet
regions (LIN and ANDERSON, 1966). They are present at all times.
0
ti
i
7.
Their origin has been attributed to surface waves on the magneto-
pause. In the past it has been tacitly assumed that these waves
are an incidental feature of these regions and play no role in deter-
mining the properties of the plasma in the regions in which they pro-
pagate. However, it appears that their role may be of great impor-
tance in the energy balance of the plasma sheet:
1. They may trigger the instabilities that give rise to the plasma
sheet acceleration events (particle islands). The plasma sheet
is a region of 0 — 1 so that a relatively small disturbance may
be all that is required to begin a transfer of magnetic energy
into particle energy (ANDERSON and NESS, 1966).
2. The high particle energies in the plasma sheet may be supplied
by the h-m waves. SAME (1968) has shown that even when the
plasma is not in its highly heated state the prrP*on temperatures
are several hundred electron volts. The energy density is
about 500 eV/cm 3 at these times and much higher when the
plasma becomes suddenly heated. It has been noted that the
trains of h-m waves are rather short-lived (LIN and ANDERSON,
1966). They appear for several cycles then disappear. If it
is assumed they dissipate their energy in the plasma (rather than
propagating indefinitely along the tail with no loss) the energy
loss per unit volume for a typical train of waves may be cal-
culated from the following formula:
B b
0 0U = 4r
where Bo is the strength of the undisturbed field and b o is the
Y
initial amplitude of the wave. For Bo
 = 15 -y and b0
 = 5 Y the
S.
energy density is 100 eV/cm 3 , on the order of the observed
plasma energy density. Further heating could b y prc,cjtic•cd
by more wave trains. The final temperature of the plasma
would then depend on how the plasma loses energy. A typical
period for the waves is 300 to 400 seconds and the wave train
may last for 5 cycles. This gives a heating rate of about
0.05 eV/cm 3 sec. Mechanisms have been proposed by which
such waves should dissipate their energy in collisionless plasmas
(BARNES, 1967)0
3.	 The east-west asymmetry in the appearance of energetic electrons
on the VELA orbit observed by BAME et al (1967) may be du
to these waves. If the wave activity is greatest on the dawn
side of the magnetotail and if the waves are indeed responsible
for heating the plasma or triggering the plasma into an instability
then energetic particles would be expected more frequently on
the dawn side of the tail. There is evidence from the 1MP-3
and EXPLORER-35 particle experiments that the energetic
electrons are more strongly modulated on the dawn than on the
dusk side. Since this modulation is believed to be due to the
presence of h-m waves, it does appear that the wave activity is
greatest on the dawn side. This might be due to the spiral
character of the interplanetary field which would then allow more
effective connections to the geomagnetic field on the dawn side
of the tail. If this is the case there should be a north-south
asymmetry since toward sectors should then connect to the
southern half of the magnetotail and away sectors to the north
magnetotail.
9.
RErERENCES
ANDERSON, K.A. , 1965, JGeophys. Res. , 7^O, 4741
ANDERSON, K. A. , HARRIS, H. K. and PAOLI, R.J. ,	 1965, J_ Geophys.
Res. , 701 1039
ANDERSON, K.A. and NESS $ N.F., 1966, J.-  Geophys. Res., 71, 3705
SAME, S.J. , 1968, Earth ' s Particles and Fields (Ed. by B.M. McCormac)
Reinhold Publishing Corporation, N.Y., 373
GAME, S. J. , ASHBRIDGE, J.R., FELTHAUSER, H. E. , HONES, E.W.
and STRONG, I. B. , 1967, J. Geophys. Res., 72, 113
BARNES, A., 1966, Phys. Fluids, _, 1483
DRIATSKIY, V.M. , 1967, Geomag. and Aero. , 7, 724
HUDSON, H.S., 1966, Energy Spectrum of Auroral Zone Electron
Prec_ipitation, University of California Ph. D. Thesis
KONRADIe A. , 1966, J. Geophys. Res., 71, 2317
MONTGOMERY, M. D. , SINGER, S. , CONNER, J. P. and STOGSDILL,
E.E. , 1965, Ph Xs. Rev, 	 14, 209
PARTHASARATHY, R. and REID, G.C. , 1967, P_ land. Space Sci. ,
917
ROTHWELL, P. , 1967, Space , Sci^Rev_. , Z. 278
VASYLIUNAS, V.M. , 1968, J. Geophys_ Res., 73, 2839
-- i	 -
10.
FIGURE CAPTIONS
Figure 1:
	
The spatial extent of the plasma sheet as determined
by VELA and OGO spacecraft. The plasma sheet
comes to the auroral regions on the night side of the
earth and the equatorial inner boundary is observed
to move toward the earth during magnetic bays.
Figure 2:	 Examples of energetic electron fluxes in the plasma
sheet 60 Re into the tail. The characteristic fast-
slow behavior is apparent. Some of the fluxes do not
seem to show this characteristic. All thv fluxes are
strongly modulated by h-m waves with periods of a
few minutes.
Figure 3: Same as Figure 2
Figure 4: Summary of the properties of the particles accelerated
in the plasma sheet. The electrons progressively soften
with time while the proton fluxes do not do so. The
pitch angle distribution is anisotropic early in the event
but soon becomes isotropic. The switching off of the.
particles which occasionally occurs is probably due to
sudden contractions of the plasma sheet.
Figure 5:	 Electron precipftatio: > into the auroral zone during sub-
storms shows the same temporal behavior as island
fluxe s.
Figure 6:	 A comparison of particle fluxes observed in the geo-
magnetic tail with riometer records.
V
N%"
11.
Figure 7:	 A comparison of particle fluxes 60 R e into the magne -
totail with ground level magnetometers. When island
events occur in the tail the station near local midnight
shows a disturbance. Conversely, when the station is
several hours from local midnight the magnetic dis-
turbance is small during island events in the tail.
Figure 8:	 The peal, flux of plasma sheet acceleration events
decreases with radial distance. The IMP-3 measure-
ments were made between 20 and 40 R while thee
EXPLORER -35 measurements came from 60 Re
geocentric distance. The frequency of occurrence of
the island events above a low flux threshold does not
vary markedly with geocentric distance.
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Relation of energetic particles in the plasma sheet to the auroral zone

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